Conventionally, carbon fibers and graphite fibers (herein referred to collectively as "carbon fibers") formed by using acrylonitrile-based fibers as precursors have excellent mechanical properties and are hence being used as fibrous reinforcements in high-performance composite materials for use in a wide range of applications including aerospace applications, as well as sports and leisure applications. In order to enhance the performance of such composite materials, it is desired to further improve the quality and performance of carbon fibers. At the same time, it is expected to reduce the production cost of carbon fibers and thereby expand their use to industrial material applications.
In contrast to acrylic fibers for clothing use, acrylonitrile-based fibers for use as precursors of carbon fibers are no more than intermediate products for the formation of carbon fibers as final products. Accordingly, it is not only desirable to provide acrylonitrile-based fibers capable of yielding carbon fibers having excellent quality and performance, but it is also very important that the acrylonitrile-based fibers have good stability during spinning of precursor fibers, exhibit high productivity in the stabilization step for forming carbon fibers, and can be provided at low cost.
From this point of view, a large number of propositions have been made in order to provide acrylonitrile-based fibers capable of yielding carbon fibers having high strength and high elasticity. These propositions include, for example, an increase in the polymerization degree of the starting polymer, and a decrease in the content of copolymerized components other than acrylonitrile. As to the spinning method, dry jet wet spinning is commonly employed.
However, when the content of copolymerized components other than acrylonitrile is decreased, the solubility of the resulting polymer in solvents is generally reduced. This not only detracts from the stability of the spinning solution, but also coagulated filament is voidful, making it difficult to form precursor fibers stably. These problems have been overcome by employing the dry jet wet spinning process.
Since the dry jet wet spinning process comprises extruding a polymer solution through a nozzle into air and then passing it continuously through a coagulating bath to form filaments, it is easy to obtain dense coagulated filaments. On the other hand, a decrease in the pitch of nozzle holes will cause a problem in that adjacent filaments may adhere to each other. Thus, there is a limit to the number of nozzle holes.
As contrasted with the dry jet wet spinning process, the wet spinning process commonly used for the production of acrylic fibers can provide such a high coagulation rate that nozzle holes can be arranged at a higher density. Accordingly, the wet spinning process has superiority from the viewpoint of productivity. For this reason, it has been eagerly desired to provide acrylonitrile-based precursor fibers which can be prepared by the wet spinning process and are suitable for the formation of high-performance carbon fibers.
However, the bundle of fibers obtained by the wet spinning process generally include many broken fibers and much fuzz. Moreover, this spinning process is characterized in that the resulting precursor fibers have a low tensile strength and a low elastic modulus, and in that the fiber structure of the precursor fibers is less dense and has a low degree of orientation of molecular chain. Consequently, the mechanical properties of the carbon fibers obtained by stabilizing them are generally unsatisfactory.
Accordingly, a number of methods for densifying the fiber structure while employing the wet spinning process have been disclosed up to the present.
For example, Japanese Patent Publication No. 39494/'79 discloses a method for forming a highly dense acrylonitrile-based fiber according to a wet spinning process using a non-aqueous organic solvent as the coagulant. However, this method is not economical in that a non-aqueous organic solvent is used in the coagulating bath.
Japanese Patent Laid-Open No. 214518/'83 discloses a precursor fiber characterized by the structure of the fiber and, in particular, the thickness of the skin layer, with the main purpose of improving its processability in the stabilization step and the quality of the resulting carbon fiber. However, no consideration is given to the polymer composition and the coagulated filament structure which are important factors governing the structure of the fiber. Accordingly, this precursor fiber is unsatisfactory from the viewpoint of improvement of the performance of the carbon fiber.
Furthermore, with respect to the acrylonitrile-based polymer used as the starting material for the formation of acrylonitrile-based precursor fibers, due consideration must be given not only to its formability into fibers, but also to complicated thermochemical reactions taking place in the stabilization step.
That is, in order to produce carbon fibers having excellent quality and performance at lower cost, it is desirable that, when acrylonitrile-based precursor fibers are converted to a carbonaceous structure by stabilization heat treatment, they scarcely produce pyrolyzates which may cause fusing of the fibers and a reduction in the performance of the resulting carbon fibers, and they have thermal reaction characteristics which permit this conversion to be effected by stabilization for a short period of time.
Since the conversion of acrylonitrile-based fibers to carbon fibers involves drastic physical and chemical changes, the causal relationship between them is quite indistinct. Although extensive investigations have been made in order to elucidate them theoretically, many problems still remain unsolved in the present situation.
There are few investigations which quantitatively show, from an industrial point of view, what is the suitable composition of the acrylonitrile-based polymer basically constituting acrylonitrile-based precursor fibers.
The findings of previous propositions can be summarized as follows. It is preferable that an acrylonitrile-based polymer for the formation of a carbon fiber precursor have a composition in which acrylonitrile units are contained in a proportion above a certain limit (not less than about 90% by weight). In order to allow precursor fibers to pass through the stabilization step in a short period of time, it is effective to introduce suitable reaction-initiating groups, i.e., functional groups accelerating the cyclic condensation reaction of the nitrile group (e.g., carboxyl groups). In addition to these conditions, other comonomers may be added in order to facilitate the formation of precursor fibers.
For example, when a polymer having a high content of acrylonitrile units in the polymer composition is used, its solubility in solvents is reduced. Consequently, the method for forming precursor fibers is very limited and, moreover, the concentration of the spinning solution is very low. Thus, this polymer is less than satisfactory from the viewpoint of carbon fiber performance and spinning formability.
When the contents of copolymerized components are increased to extend latitude in spinning-forming, precursor fibers formed from this polymer tend to fuse together during stabilization heat treatment and, moreover, show a reduction in carbonization yield. Thus, this polymer is still unsatisfactory from the viewpoint of processability in the stabilization step and the quality and performance of the carbon fibers.
In order to overcome these various problems and, at the same time, suggest the composition of starting polymers which can be fired and carbonized in a short period of time or are advantageous for this purpose, the following propositions have been made.
For example, there have been proposed a process wherein an improvement in stabilization rate and carbonization yield is achieved by using a polymer composition having high reactivity for cyclization and oxidation in the stabilization (Japanese Patent Publication No. 33019/'72); a process wherein the polymer composition is specified (e.g., by the use of a vinyl carboxylate monomer) so as to reduce the stabilization time with consideration for stability in the polymer preparation and spinning steps (Japanese Patent Laid-Open No. 7209/'76); and a process wherein an amine salt or a peroxide is added to the starting polymer (Japanese Patent Publication No. 7209/'76 and Japanese Patent Laid-Open No. 87120/'73).
However, all of these patents simply present a broad range of constitution with respect to the polymer composition (i.e., the types and contents of the copolymerized monomers), and cannot possibly be said to disclose a well-defined composition which satisfactorily exhibits the properties (e.g., the behavior of stabilization) required of precursor fibers. Moreover, although the acceleration of the stabilization reaction itself is believed to enable high line-speed, this rather tends to detract from the performance of the resulting carbon fibers. Thus, it is impossible to achieve an improvement in both the productivity and the performance of carbon fibers. Furthermore, the addition of an amine or a peroxide to the polymer exerts various adverse effects on the stability of the spinning solution and the precursor fibers, and cannot be regarded as an industrially excellent method.
Meanwhile, Japanese Patent Laid-Open No. 34027/'77 discloses a process wherein a high-performance carbon fiber can be economically and stably produced by specifying the composition of a polymer and modifying the conditions of stabilization treatment. In particular, it is worth noting that the combined use of (meth)acrylamide and a carboxyl-containing monomer is uniquely effective in accelerating the stabilization reaction.
Moreover, Japanese Patent Laid-Open No. 339813/'93 proposes a process wherein a highly dense acrylonitrile-based precursor fiber is obtained by controlling the composition of a copolymer comprising acrylonitrile, acrylamide and methacrylic acid, and subjecting this copolymer to wet spinning. This proposition has made it possible to make up for the shortcomings of conventional wet spinning processes. However, this acrylonitrile-based precursor fiber is still unsatisfactory for the purpose of producing a carbon fiber having higher performance.
Thus, although many processes have heretofore been proposed, there has not yet been obtained an acrylonitrile-based precursor fiber for the formation of a carbon fiber which has high productivity and can yield a high-performance carbon fiber. In particular, many propositions on the composition of an acrylonitrile-based polymer have been made in order to carry out the stabilization reaction efficiently in the stabilization step, whereas no attempt to control the fiber structure in the coagulating step governing the fiber structure and thereby obtain an acrylonitrile-based precursor fiber for the formation of a high-performance carbon fiber has been proposed in the present situation.